Compresion test

[Hotrods]

I own a 1987 80 hp mercury outboard. with very low hours, I am in the process of selling it. I asked in the forum before what the compresion should be and I was told 90. I ran the test and got 60 on all three, was told as long as they are all the same it's good. so is this good? should it be 90. does 60 show wear? Thanks in advance I know I came to the right place<br /><br />Rod

 

[NathanY]

Re: Compresion test

90 is not bad but 100 is better. Personally I would not buy a motor with less than 100lbs. If I where you I would try a good decarb like Mercury Power Tune, and see if you can get those numbers up.

 

[emckelvy]

Re: Compresion test

60 is hard to believe, it would barely run.<br /><br />Are you using a good quality compression gage. Some of the lesser varieties do not give accurate readings.

 

[Hotrods]

Re: Compresion test

This motor runs great! as good as new. yea im using a good tester, and only allowing the neddle to move once, i did get a 100 reading but i thought maybe it cranked twice. Im a Noob! at this.<br /><br />After doing some searching I do think I muffed the test. Could someone give me the proper steps, do I warm the engine,? how many time can i crank it, do I open the throttle all the way to let air in?....thanks all again<br /><br />Rod

 

[NathanY]

Re: Compresion test

To get an accurate compression reading you have to crank the motor until the compression guage peaks out. Cranking over one time will not give you a compression reading.

 

[andrewkafp]

Re: Compresion test

Doesn't matter how many times it cranks.You can even put the comp tester on a running motor and you will only get the maximum that the cylinder will compress. That motor should be delivering 120psi minimum. 90 60 100 are low.

 

[Hotrods]

Re: Compresion test

great ...let me go check..

 

[Hotrods]

Re: Compresion test

ok, got an even 110 on all 3, cold, would warming up the engine raise this, so whatdu yu think? does this show wear on the motor, dang i havent used it that much and it was well maintained..

 

[andrewkafp]

Re: Compresion test

110 is fine..My 50hp is around 105 in all 4 (cold) and runs at around 25 knots at 4500.. Consistency is the key.

 

[Hotrods]

Re: Compresion test

So do you think having 110 compresion in all 3 is a good selling piont . I am claiming motor has minimal use, that is does.

 

[jfholijr]

Re: Compresion test

I wouldn't use it as a selling point because mine and many others I know of have around 150 psi on each cylinder. If it is still on the boat then whoever is interested in buying it, just take them for a test run.

 

[jimalsk]

Re: Compresion test

I have a '85 75hp merc with a compression of 141-139 on all 4 cylinders. I also own a older model 50hp merc with 125 in all cylinders. 60 seems to be a bit low. Are you cranking the engine till the tester can go no higher?